A Balanced Vortex approach to Improving the Initial Condition and Diagnosing Forecasts in the Hurricane Research Weather and Forecasting Model

The axi-symmetric component of the tropical cyclone (TC) wind field has been shown via numerical methods and observations to be well approximated by thermal wind balance throughout much of the vortex lifetime. When this assumption is valid, it constrains vortex evolution by requiring a secondary overturning circulation to develop in response to forcing due to diabatic heating in the eyewall and surface friction. This secondary circulation acts to restore a balanced state through the redistribution of mass and momentum and therefore has a significant role in determining whether the TC will strengthen. It then follows that an accurate representation of this circulation in numerical hurricane model initial conditions is essential for obtaining the proper tendencies for intensity forecasts. We hypothesize that too weak of a secondary circulation in the initial condition may lead to a spinning down of the model vortex in the short term, negatively impacting the forecast of intensity. In this study we apply the concept of a balanced vortex to evaluate the initial condition in the Hurricane Weather Research and Forecasting Model (HWRF) which was derived using the Hurricane Ensemble Data Assimilation System (HEDAS) to assimilate observations taken within the TC inner core. First we test the validity of the balance approximation by comparing the axi-symmetric component of the model wind, temperature and pressure fields with gradient and hydrostatic balance. We then solve a diagnostic equation to obtain the overturning secondary circulation which corresponds to the diabatic heating determined from the model. The initial and evolving circulations in the model are evaluated relative to the balanced solution in order to determine whether a more balanced state corresponds to the vortex ability to maintain its initial intensity. If this is the case, the balanced solution could potentially be incorporated into the initial condition in order to mitigate vortex spin down in the forecast.